1. Field of the Invention
The present invention is directed to a small single lens reflex camera of the 110 film format variety and more particularly to an improved view finder optical system that maintains the compact size of the camera body.
2. Brief Description of the Prior Art
The introduction of a 110 film format into the commercial market in 1972 excited an interest in attempting to achieve the versatility of a 35mm single lens reflex (SLR) with the appeal of a pocketable camera. The now famous Kodak INSTAMATIC pocket camera accompanied the introduction of the 110 film cartridge but was limited in providing quality photography.
A commercial reality in the photography field is the domination of film manufacturers in setting film parameters which must be adhered to in the designing of a compatible camera. In this regard, the 110 film cartridge established the minimum dimensions of the 110 camera, this also placed a limitation on the location and size of a mirror in a 110 SLR camera. The film format size of 13 .times. 17 millimeters placed additional limitations on the possible camera optical systems.
One of the stringent demands placed on the designing of a 110 SLR camera is the high requirement of accuracy in the focusing system. This accuracy requirement is about twice as stringent as that for a 35mm SLR camera because the 110 film image must be enlarged about twice that of a 35mm to obtain an equal size final image. Generally, a quality 35mm SLR camera will specify an optical path tolerance between the focusing plane and the film plane of a .+-.0.02mm. An equal picture quality 110 SLR camera will have to provide for a plus or minus tolerance of 0.01mm. The viewfinder system being of critical importance since any focusing errors will be enlarged when the final image is prepared.
Another stringent demand on the focusing system of a 110 SLR camera occurs when the focusing is also utilized to adjust the lens aperture for proper exposure. Since the speed of most of the film of a 110 format, e.g., ASA 64-80, is limited, the use of supplemental flash lighting is important. Thus, the accuracy of focusing can be important in determining the correct flash exposure in a 110 SLR camera.
The advantages of an SLR camera along with the portability and convenience of a 110 cartridge camera size demands a more stringent focusing system while still maintaining the compact camera body.
An additional problem in designing a competitive compact 110 SLR camera is manufacturing cost. Thus, not only a small lightweight compact design must be provided, but both improved focusing capabilities with an economical manufacturing procedure must be provided to have a successful commercial product.
Various viewfinder optical systems are known in single lens reflex cameras such as the use of reflecting surfaces in a roof configuration to provide a viewfinder image that is not inverted. Generally, 35mm SLR viewfinder optical systems seeking to use a roof reflector configuration will utilize a prism having a pentagonal cross-section. Viewfinder optical systems, however, using a penta-prism are generally heavy and further difficult to manufacture. These design and cost impediments are particularly unacceptable in a compact 110 SLR camera.
Suggestions have been made to overcome the shortcomings of a penta-prism viewfinder system by the use of a plurality of reflecting mirrors to form a porro-prism optical system. Problems exist with this design in providing the spacing necessary between the reflecting mirrors. These problems are again compounded in attempting to maintain the compact size of a 110 SLR camera.
Thus, the conflicting design parameters associated with a commercial 110 SLR camera are still being sought to be satisfied by the prior art.